The genetic relationship between andesites and dacites at Tungurahua volcano, Ecuador

2018

Article référencé dans le Web of Science WOS:000424067500019

Nauret F., Samaniego Pablo, Ancellin M. A., Tournigand P. Y., Le Pennec Jean-Luc, Vlastelic I., Gannoun A., Hidalgo S., Schiano P.

Journal of Volcanology and Geothermal Research, 2018, 349, p. 283-297 ISSN 0377-0273

Volcanic eruptions of intermediary and silica-rich magmas (andesites, dacites and rhyolites) in convergent arc settings generate voluminous and explosive eruptions that can strongly affect human activity and have significant environmental impacts. It is therefore crucial to understand how these magmas are generated in order to anticipate their potential impact. At convergent margins, primitive magmas (primitive basalts and/or andesites) are derived from the mantle wedge and they are progressively modified by physical and chemical processes operating between the melting zone and the surface to produce silica-rich magmas. In order to elucidate the relationship between andesites and dacites, we focus on Tungurahua volcano, located in the Ecuadorian Andes. We collected a set of samples comprising such lithologies that were erupted during the last 3000 year BP. This relatively short period of time allows us to assume that the geodynamic parameters remain constant. Petrology and major-trace element compositions of these lavas have already been examined, and so we performed a complementary Pb-Sr isotope study in order to determine the nature and origin of the components involved in andesite and dacite genesis. Sr isotopes range from 0.70417 to 0.70431, and Pb isotope compositions range from 18.889 to 19.154 for Pb-206/Pb-204, from 15.658 to 15.696 for Pb-207/Pb-204, and from 38.752 to 38.918 for Pb-208/Pb-204. Dacites display a remarkably homogeneous Pb isotopic composition, with higher Pb-206/Pb-204 values for a given Pb207-208/Pb-204 compared to andesites. Andesites show notable Pb-207/Pb-206 variations for a given SiO2 content, whereas dacites have lower and homogenous Pb-207/Pb-206 values. Andesite and dacite altogether plot in a roughly triangular distribution, with dacitic magmas systematically plotting at the high SiO2 and Sr-87/Sr-86 and low Pb-207/Pb-206 fields. Based on our new dataset, we show that at least 3 different components are required to explain the Tungurahua compositional and isotope variation: one corresponds to the mantle, the second has a deep origin (slab component or lower crust), and a mixture between these two components explains andesite heterogeneity. The third component is derived from the underlying upper continental crust. While andesites are derived from deep components, dacites are derived from the andesitic magmas that underwent an assimilation-fractional crystallization (AFC) process with incorporation of the local metamorphic basement. Finally, we used the geochemical and isotopic data to produce a model of the magmatic plumbing system beneath Tungurahua, consistent with geophysical and experimental petrology constraints. We conclude that melt migration and storage in the upper crust appears to be a key parameter for controlling volcanic behavior though time.

Géologie et formations superficielles [064] ; Géophysique interne [066]

EQUATEUR ; TUNGURAHUA VOLCAN

Fonds IRD [F B010072326]

fdi:010072326